Such expressions as that famous one of Linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications (Darwin, 1859, p. 413f).

Friday, 26 March 2010

The Mystery of the Siberian Pinkie: When Classification goes Wrong

ResearchBlogging.orgBiological classification is often misunderstood and misused in the scientific literature and especially in the media:
    “... birds are dinosaurs” (New York Times, 2010).
    “Humans are apes” (Dawkins, 2010 in The Australian).
    “Scientists have identified a previously unknown type of ancient human through analysis of DNA from a finger bone unearthed in a Siberian cave” (BBC News Online).
These slogans, popular with the public, are completely misinformed and provide inaccurate information about biological classification. Let’s start with birds are dinosaurs.

The slogan assumes we know something about the classification of dinosaurs, namely that they include birds. This assumption derives from a cladogram that shows that some dinosaurs (feathered therapods) are more closely related to birds than they are to other dinosaurs (see Gauither et al., 1998).

Moreover, birds are diagnosed as any amniote that possesses feathers, meaning that therapods are birds. The slogan then should instead read “some dinosaurs are birds”. This then begs the question “what are dinosaurs?”

The phrase “Humans are apes” has been used quite often as a slogan, most notably by Richard Dawkins. Like the dinosaur bird example, it assumes that we know something about human classification. In fact, the term ‘ape’ refers to:
    “... any of two families (Pongidae and Hylobatidae) of large tailless semi-erect primates (as the chimpanzee, gorilla, orangutan, or gibbon) ...” (Merriam-Webster Online Dictionary, 2010).
Humans on the other hand belong to the Hominidae. In classification and in common usage, we are hominids and not apes. So why the confusion?

Many scientists misuse existing terms in classification to make assumptions about evolution. Since evolution and classification are two separate issues, there is a degree of confusion. The slogan “humans are apes” is supposed to infer that humans evolved from apes or both apes and humans evolved from something else. In classification, this slogan is meaningless, unless it is meant to say that humans are members of either pongidae and hylobatidae. The same misuse of classification can be seen in a recent report of a newly discovered hominid from Siberia. The report by Krause et al (2010) states:
    “DNA sequence retrieved from a bone [the distal manual phalanx of the fifth digit, or the 'pinkie'] excavated in 2008 in Denisova Cave in the Altai Mountains in southern Siberia. It represents a hitherto unknown type of hominin mtDNA that shares a common ancestor with anatomically modern human and Neanderthal mtDNAs about 1.0 million years ago (Krause et al., 2010).
mtDNA is not used in taxonomy to diagnose new species. Moreover the variability in mtDNA has been found to be high between individuals (see He et al., 2010). So what is this hullabaloo about? It appears it concerns itself over a newly discovered pinkie and there isn't much it can tell. You need more evidence than a pinkie to determine and describe a new species into an existing taxonomy. What Krause et al (2010) have found is a reconfirmation that hominid mtDNA is highly diverse. But yet, reading both Krause et al (2010) and the media, we are led to believe that a new species of hominid has been discovered. This is clearly not the case. Until further evidence is found (e.g., bones or bone fragments) and a diagnosis made can we be sure that “scientists have identified a previously unknown type of ancient human”.

References

Gauthier, J. A., Estes, R and de Queiroz, K 1988. A phylogenetic analysis of Lepidosauromorpha. In: R. Estes and G. Pregili (eds), Phylogenetic relationships of the lizard families. Stanford University Press, Stanford, CA, pp. 15-98.

He, Y., Wu, J., Dressman, D., Iacobuzio-Donahue, C., Markowitz, S., Velculescu, V., Diaz Jr, L., Kinzler, K., Vogelstein, B., & Papadopoulos, N. (2010). Heteroplasmic mitochondrial DNA mutations in normal and tumour cells Nature, 464 (7288), 610-614 DOI: 10.1038/nature08802

Krause, J., Fu, Q., Good, J., Viola, B., Shunkov, M., Derevianko, A., & Pääbo, S. (2010). The complete mitochondrial DNA genome of an unknown hominin from southern Siberia Nature DOI: 10.1038/nature08976

10 comments:

Lucas Brouwers said...

I don't think Mirriam-Webster is a good resource for taxonomic definitions. NCBI taxonomy lists apes(Hominoidea) as a monophyletic group, including humans. So humans are indeed apes, just as much as we are also primates, mammals and vertebrates.

Carl said...

I'm the reporter who wrote the dinosaur story in the Times you linked to. Birds are not diagnosed by feathers, despite your contention. Readers who want to see the current consensus on the relationship of birds to other dinosaurs should consult an expert like Thomas Holtz of the University of Maryland: http://www.geol.umd.edu/~tholtz/G104/104HOs.html

John S. Wilkins said...

My reading here is that the author is saying that vernacular terms are not taxonomic names. This seems right – if we don't think that reptiles are a monophyletic group if they don't include birds, why should we think "birds" = "Aves" or "ape" = "Hominoidea"? There is a fallacy of ambiguity here.

Also, the author is a pattern cladist, or transformed cladist (I forget which) and thus (in my view mostly correctly) thinks that inclusion of a taxon within another taxon in a cladogram is not prima facie evidence that the one is a descendent from the other.

David Williams & Malte Ebach said...

With respect to “Birds are Dinosaurs”, we wrote:

“Moreover, birds are diagnosed as any amniote that possesses feathers, meaning that therapods are birds. The slogan then should instead read "some dinosaurs are birds". This then begs the question "what are dinosaurs?"


Birds have feathers; some therapods have feathers; so it is not 'dinosaurs' that have feathers nor that birds are 'dinosaurs'. Only "Some dinosaurs are birds". So the question remains: What are dinosaurs? There is no answer.

Augray said...

"Birds have feathers" is not particularly useful, since a) this doesn't mean that non-birds can't have feathers, and b) the definition of "feathers" in this context is uncertain. Using a generous definition of "feathers", one might argue that pterosaurs are birds.

And in answer to the question "What are dinosaurs?", the author seems to agree that they're amniotes, so claiming that "There is no answer" seems contradictory.

Bjørn Østman said...

So the question remains: What are dinosaurs? There is no answer.

This is clearly not true, though, as much as I agree that there need not be strict agreement between scientific nomenclature and the vernacular.

According to Wikipedia (which I think we should all trust more on these matters than Merriam-Webster, but do go ahead and shoot me):

Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of "Triceratops, Neornithes [modern birds], their most recent common ancestor, and all descendants."[10]

Scientifically, "dinosaurs" works, and so saying that birds are dinosaurs is correct. Vernacularly the term may be muddled (i.e. paraphyletic), but who cares? Same goes for rats/mice and so many other species.

Michael said...

"... inclusion of a taxon within another taxon in a cladogram is not prima facie evidence that the one is a descendent from the other"

Did you mean 'definitive' rather than "prima facie"?

Michael said...

My mind has dredged up a dim memory of a discussion of this point in which phylogenetic description, in which 'humans are fish' makes sense and the ecological, in which 'humans are fish' doesn't. Our vernacular would be rooted in the ecological, since our environment is where humans first encountered most major groups of organisms. Not sure that helps anyone but me to understand the distinction.

Anonymous said...

Think again.

"We (humans) are apes" or "we (birds) are dinosaurs" might seem reasonable. How about:

We (mammals) are reptiles? or
We (vertebrates) are invertebrates?

In general this kind of statement does not work. Why not?

If it does not work in general, how can it work in a specific case?

The other Jim said...

To anonymous,

We have a long history of making classifications that do not stand up as the data accumulate. The classifications of "reptile" and "invertebrate" are two examples. Common use of the terms still persists, but "all animals except vertebrates" and "all terrestrial vertebrates except birds, amphibians, and mammals" are no longer meaningful divisions.

Refer back to John Wilkins post re: vernacular terms versus taxonomic names.